Comparative conformational analysis and in vitro pharmacological evaluation of three cyclic hexapeptide NK-2 antagonists

The conformational analysis of three cyclic hexapeptides is presented. Cyclo‐(‐Gln6‐Trp7‐Phe8‐Gly9‐Leu10‐d‐Met11‐) (1) and cyclo‐(‐Gln6‐Trp7‐Phe8‐Gly9‐Leu10‐Met11‐) (2) are NK‐2 antagonists in the hamster trachea assay, whereas cyclo‐(‐Gln6‐Trp7‐Phe8‐(R)‐Gly9‐[ANC‐2]Leu10‐Met11‐) (3), where Gly9[ANC‐2]Leu10 represents (2S)‐2‐((3R)‐3‐amino‐2‐oxo‐1‐pyrrolidinyl)‐4‐methylpentanoyl, is inactive as agonist and antagonist in this assay. In DMSO, the NMR results cannot be interpreted as being consistent with a single conformation. However, the combined interpretation of results from NMR spectroscopy, restrained molecular dynamics simulations with application of proton–proton distance information from ROESY spectra, and pharmacological results leads to a reduced number of conformational domains for each peptide, which can be compared with each other and may be classified as responsible for their biological activity. Trying to match the conformational domains approximately with regular β‐ and γ‐turns, we find a γn‐turn at the position of the methionine occuring in all peptides. For the active peptides 1 and 2 we arrive at an inverse γi‐turn at Phe8, and βI′‐ or βII‐turns with Gly9 and Leu10 at the corner positions, these β‐turns having a similar topology with respect to the linking peptide unit. Other conformational domains common to only 1 and 2 support their classification as responsible for the biological activity.